During the assembly of components on printed circuit wiring boards, soldering fluxes are first applied to the substrate board material to insure that the solder will firmly bond the components to the wiring traces imprinted on the printed circuit board. Generally, these fluxes are either rosin-based or non-rosin-based fluxes. Non-rosin-based fluxes are generally water soluble and removal does not cause any major problems.
Rosin-based fluxes, however, because of their reliability, are used for military applications and other areas where greater reliability is required. These rosin-based fluxes are not easily removed with water alone. Typically, these fluxes are removed with the use of solvents. Chlorinated hydrocarbon solvents have been found to be useful for this purpose but these solvents are toxic and not readily bio-degradable, and are, therefore, hazardous for a long period of time. Fluorocarbon solvents have also been found useful for this purpose. These solvents, however, when released to the atmosphere, destroy the ozone layer in the atmosphere and are, therefore, no longer preferred. Haves, et al., U.S. Pat. Nos. 4,640,719 and 4,740,247, disclose a new terpene solvent which is suitable for removing rosin-based fluxes from printed circuit boards. Unfortunately, terpene solvents are combustible.
Apparatus to contain and apply solvents for cleaning printed circuit boards can be generally classified into continuous systems and batch systems. Kishida, et al., U.S. Pat. No. 4,589,926, discloses a linear continuous system for cleaning printed circuit boards. The printed circuit boards are carried by a cassette which is placed in one end of the apparatus and advanced through zones within the apparatus. In one of the zones, liquid solvent is sprayed on the printed circuit board while brushes simultaneously scrub the board. The solvent is collected, vaporized, and distilled to remove impurities prior to being reused. Although this apparatus is suitable for the particular solvent used, namely "Arklone.RTM.", a carbon fluoride 113, it is not practical for combustible solvents because solvent vapors can still enter the atmosphere from the entrance or the exit end of the apparatus, creating a fire hazard.
Batch systems have been devised such as shown in Kishida. et al., U.S. Pat. No. 4,643,774, for cleaning substrate. Kishida, however, immerses the substrate into an open wash liquid container. This allows wash liquid vapors to enter the atmosphere. Drying air is also directed at the substrate as they are removed from the wash liquid container, further forcing wash liquid vapor into the air. Kishida, therefore, would not be useful for combustible or toxic solvents.
Conventional commercial dishwashers have been used to remove water-based fluxes from printed circuit boards. Commercial dishwashers are not readily adaptable to use combustible solvents for cleaning the boards because of the potential of explosion and the adverse effect the solvents have on the interior and the seals used in the conventional washing machine.
Therefore, a need exists for an improved batch cleaning system for printed circuit wiring boards which can safely use combustible solvents for cleaning the boards.